1. Field of Invention
This invention relates to a failure detection apparatus and a failure detection method for an exhaust gas sensor and, more particularly, to a failure detection apparatus for an exhaust gas that detects cracks in the exhaust gas sensor, and a method for detecting a failure in the exhaust gas sensor.
2. Description of Related Art
One system for detecting abnormalities in an oxygen sensor disposed in the exhaust passage of an internal combustion engine is described in Japanese Patent Application Publication No. JP-A-HEI-08-327586. The reference describes an oxygen sensor with exhaust-side electrode that is exposed to exhaust gas, and an atmosphere-side electrode that is exposed to an atmosphere layer within a sensor element thereof.
The atmosphere layer is a space that is separated from the interior of the exhaust passage by the sensor element. In addition, the atmosphere layer communicates with the atmosphere. The described system applies a voltage between the atmosphere-side electrode and the exhaust-side electrode, and then determines whether an abnormality is present based on the magnitude of the electric current that flows between the electrodes.
In the described system, the exhaust-side electrode serves as a positive electrode and with the atmosphere-side electrode serves as a negative electrode. When a voltage is applied between the two electrodes, an electric current corresponding to the oxygen concentration in the atmosphere layer flows through the sensor element. That is, if the sensor element is normal, the electric current corresponding to the oxygen concentration in the atmosphere (hereinafter, referred to as “normal electric current”) flows. On the other hand, if there is a crack in the sensor element and exhaust gas enters the atmosphere layer, the oxygen concentration in the atmosphere layer is decreased, which results in a decrease in the electric current that flows through the sensor element relative to the normal electric current.
Thus, the value of the electric current that is generated by applying voltage between the exhaust-side electrode and the atmosphere-side electrode changes in accordance with whether or not there is a crack in the sensor element. Therefore, by focusing attention on the value of the electric current, it is possible to determine whether or not there is a crack in the sensor element.
However, the magnitude of the electric current that flows through the sensor element changes in accordance with the impedance of the sensor element. The temperature of the sensor element affects the impedance of the sensor element. Therefore, unless the temperature of the sensor element is accurately detected, it is difficult to correctly determine whether an abnormality is present in the sensor element based on the electric current that flows through the sensor element. In this respect, the above-described system is not necessarily capable of correctly detecting a the presence of a crack in the sensor element at all times.